1. Field of the Invention
The present invention relates to an electrode for semiconductor devices, its fabrication method, and a sputtering target for forming an electrode film for semiconductor devices, and particularly to an electrode for semiconductor devices which is suitable for an electrode (interconnections and electrode itself) of an active matrixed liquid crystal display having a thin film transistor, and its fabrication method.
2. Description of the Related Art
A liquid crystal display (hereinafter, referred to as “LCD”) is excellent in thinning, lightening and power-saving compared with conventional displays using a cathode-ray tube, and further it is capable of obtaining a high resolution image. Furthermore, to improve the image quality, there is proposed an LCD with a thin film transistor (hereinafter, referred to as “TFT”) as a switching element. Here, the TFT means an active element composed of a semiconducting film formed on an insulating substrate such as glass, to which an electrode made of a thin metal film (interconnection and electrode itself) is connected. An electrode for semiconductor devices is used as part of a TFT, and the term “electrode” as used herein is intended to refer to both interconnections and the electrode itself. In the TFT, the interconnections and the electrode are electrically connected to each other.
Various properties are required for an electrode for semiconductor devices used for the LCD mentioned above. In the tendency toward larger display size and higher resolution of LCDs, particularly, the lowering of the resistivity becomes most important for suppressing the delay of a signal. For example, in a color LCD with more than 10-inch large display, the resistivity (electrical resistance) of an electrode for semiconductor devices is required to be lower than 20 μΩcm.
Refractory metals such as Ta, Mo, Cr and Ti are used as the electrode materials for LCDs with TFTs (hereinafter referred to as “TFT-LCD”). However, the metals have high resistivities in the thin film state; about 180 μΩcm (Ta), about 50 μΩcm (Mo), about 50 μΩcm (Cr), and about 80 μΩcm (Ti). The resistivities of all these metals greatly exceeds the value of 20 μΩcm. Accordingly, to achieve a larger size and higher resolution of TFT-LCDs, there has been required a new electrode material for semiconductor devices having a low resistivity (lower than 20 μΩm).
The electrode material for semiconductor devices having a low resistivity may include Au, Cu and Al. Au is difficult to etch, which property is required to form a specified pattern after deposition of the film, that is, electrode film, and it is expensive. Cu is poor in its adhesiveness to substrates and in corrosion resistance. Both metals are not practical. On the other hand, Al is insufficient in thermal stability, and has a disadvantage in generating fine protrusions called hillocks on the surface of an electrode film during a heating process after deposition of the electrode film which is inevitable for the TFT fabrication process. In general, in the TFT-LCD, the electrode film becomes the bottom layer, so that when hillocks are generated, it is impossible to deposit a film thereon.
To suppress the generation of hillocks on en Al electrode film, there has been adopted a technique wherein the heating is performed after a high strength film, such as the refractory metals, is deposited on the Al electrode film. However, in this technique, films with different etching properties must be simultaneously etched, so that it becomes difficult to obtain a good interconnection pattern. Accordingly, there has been required an electrode material for semiconductor devices used for TFT-LCDs capable of suppressing the generation of hillocks and reducing the resistivity (lower than 20 μΩcm).
Although the present state (prior art, problem and requirement) of an electrode for semiconductor devices of TFT-LCDs has been described, the electrode for semiconductor devices is used not only for the TFT-LCD, but also for the electrode and interconnections of an Si semiconductor represented by a Large Scale Integrated Circuit (hereinafter referred to as “LSI”). Problems in the electrode used for the LSI is the same as that of the TFT-LCD. Accordingly, there has also been required an electrode material for semiconductor devices used for LSIs capable of suppressing the generation of hillocks and reducing the resistivity (lower than 20 μΩcm).